disconnect: Finish pending work before disconnecting MQTT
disconnect: Finish pending work before disconnecting MQTT

Some pending work may require MQTT and should be dealt with before MQTT is
disconnected.

Signed-off-by: Ben Astell <ded3fae2663d1648fc922f7692e7914ea175e4a7@windriver.com>

rcwoolley/device-cloud-python,rcwoolley/device-cloud-python